1. Field of the Invention
This invention relates to laser systems, such as a frequency doubled Nd:YAG laser, operable in multiple pump power modes, and to such laser systems wherein the average power consumption of the laser is reduced so that the laser an be air-cooled while the average output power from the laser is maintained at or increased over the average output power of a conventional laser.
2. Description of the Prior Art
Lasers are now commonly used for a variety of medical, surgical, and industrial applications. Several types of solid state lasers are used in these applications, such as the Nd:YAG laser with a primary wavelength of 1064 nm in the near infrared. Also, such systems have been used with a non-linear crystal (such as a potassium titanylphosphate (KTP) crystal) inside the laser resonator, with output coupling devices designed to extract an output beam at a frequency, such as a second harmonic, derived from resonating beam. One useful system uses this technique to generate a green 532 nm output with a 1064 nm Nd:YAG laser. Typically, such Nd:YAG laser systems with a non-linear crystal, have a Q-switch to improve the conversion efficiency from 1064 nm to 532 nm.
A prior art surgical laser system produces an output beam with about 20W output power at 532 nm, which output beam is delivered to a patient's tissue through an optical fiber or other delivery system. The main advantage of the 532 nm wavelength is that it is strongly absorbed by the hemoglobin in blood and hence useful for cutting, vaporizing, and coagulating vascular tissue. Such a high power, frequency doubled Nd:YAG laser suitable for such uses is described by P. E. Perkins and T. S. Fahlen in JOSA, 4, pp. 1066-1071 (1987), and further improvements are described in U.S. Pat. No. 4,907,235, issued to Dirk J. Kuizenga on Mar. 6, 1990.
A surgical laser system typically has three modes of operation, called for identification herein, the standby mode, the ready mode, and the work mode. The laser goes to the standby mode, with no output power coming out of the system, when it is first turned on. Typically, the laser is in standby mode while the operating room is prepared and, in some cases, the laser can be in the standby mode for several hours. In this mode, the pump energy source, such as an arc lamp, may be turned on but operated below the laser threshold to warm up the pump energy source and prepare the system for an application of the work beam.
The ready mode is used to prepare the system for a fast transition to the work mode. In this ready mode, the pump energy source is driven well above laser threshold at or near the full power to be used in the work mode, to thermally stabilize the laser, so that on transition to the work mode, no undesirable transients in laser operation occur. Also, an aim beam is generated for aiming the work beam.
In a prior art frequency double Nd:YAG system, the laser gain medium is pumped at a high power during the ready mode, to generate a stable beam. The aim beam in such system is then produced by attenuating the laser output. For instance, the aim beam is obtained in a prior art frequency doubled Nd:YAG system by attenuating a laser output beam to less than 4 mW, at which the second harmonic beam is very clearly visible on vascular tissue, but not effective otherwise. Alternative ways to generate an aim beam include using other visible lasers, like a He-Ne laser at 633 nm, visible diode lasers at other wavelengths, or in some systems, using a white light incoherent source, which can be switched into the beam delivery system.
In the work mode full laser power is available to either cut, coagulate, or vaporize the target tissue. The system operator sets the required output from the laser for a typical application. In a typical system, output power can be adjusted from 50 mW to 20W.
The frequency doubled laser should switch from the ready mode to the work mode upon demand. For instance, in a surgical application, when the surgeon steps on a footswitch while aiming the beam to enable the work mode, the surgeon expects that the work mode laser output will be achieved immediately. A perceptible delay due to transients in the laser is not acceptable. Delay of more than the reaction time of the surgeon, 500 milliseconds for example, may make it difficult to insure that the work will be accomplished at the place identified by the aim beam before the switch is thrown.
The thermal effects of pumping the laser gain medium in many systems determine, to a large degree, the parameters of the ready and work modes. For instance, with Nd:YAG and other solid state gain media, thermal focusing is of concern. The pump power source, such as an arc lamp, pumps energy into the gain medium rod almost uniformly. The rod is cooled with water on the outside surface and consequently a thermal gradient is induced in the rod, with the maximum temperature at the center of the rod and lowest temperature at the outside surface where it is in contact with the water. This temperature gradient produces a thermal lens in many solid state media, and the dependance of the focal length of this lens on the laser pump power complicates the design of the laser. Thermal focusing is described in detail by W. Koechner in Applied Optics. 9, pp. 1429-1434, and pp. 2548-2553 (1970). U.S. Pat. No. 4,907,235 also discusses the design of the laser with thermal focusing. Two significant aspects of thermal focusing should be considered: first, these lasers are designed to have a stable optical resonator over a limited range of thermal focusing. At the low level end of pumping, the laser has weak thermal focusing and this may mean that the laser resonator is not stable. In that case, some minimum amount of pump power (possibly significantly higher than the theoretical laser threshold) is required to provide sufficient thermal focusing to make the resonator stable. In general, this means that more pump power has to be used to get stable and reliable output from the laser. Second, in a typical system, when the pump source is turned on rapidly from a very low level to the full power level required for stable laser operation, it takes from a fraction of a second to a few seconds to reach stable thermal conditions in the laser. During this time the output power from the laser may be erratic.
Another thermal property of Nd:YAG and some other solid-state laser materials is that the gain decreases as the temperature of the laser material increases. In a typical Nd:YAG laser system, for example, the output power starts to drop significantly due to this drop in gain as the laser cooling water temperature goes above 80 to 90.degree. F.
U.S. Pat. No. 4,907,235 discloses a frequency doubled laser having a region of stability where the laser is Q-switched at 25 kHz (40 .mu.s between pulses). In this laser, the ratio of spotsizes in the Nd:YAG medium to the non-linear crystal is about 2.5, and the laser must be pumped well above threshold with an electrical pump power of about 3 to 4 kW. Thus, the laser must be operated well above threshold all the time to get stable frequency doubled output.
Also, in prior systems, the output of the laser using non-linear crystals cannot be adjusted over a wide enough range for surgical applications by changing the pump power, such as by changing current through the arc lamp of the laser. An external laser beam attenuator is required to adjust the laser output to the range required for surgical applications. This is very different from the Nd:YAG lasers without non-linear crystals and without Q-switching, in which the output is adjustable directly with the pump power.
Other components of frequency doubled systems are also involved in the resonator design. The non-linear crystal (e.g., KTP crystal) used for frequency doubling has non-linear behavior such that the output power at the second harmonic increases with the square of the input power at the fundamental frequency, or EQU P.sub.2 =kP.sub.1.sup.2
Where
P.sub.2 =Power at second harmonic (532 nm) PA1 P.sub.1 =Power at fundamental (1064 nm) PA1 k=constant of proportionality.
This is described in several standard text books such as The Principles of Non-Linear Optics by Y. R. Shen, John Wiley & Sons, 1984, p. 86. The above equation is correct as long as the total conversion from fundamental to second harmonic remains small, typically less than 20 to 30%. The significance of the non-linear behavior becomes very important when the fundamental power is being pulsed. Consider the simple example where the fundamental power is pulsed at a 50% duty cycle and the average fundamental power remains the same. For the 50% of the time that the fundamental power is turned on, the peak power is near twice the average to maintain a constant average power. The second harmonic generation increases to four times the power that it would be with a constant fundamental input power, and for the 50% duty cycle, the average second harmonic power is increased by a factor of two. This relationship can easily be extended to show that for a duty cycle of K (fraction of the time the power is turned on) and with the average pump power the same, the average second harmonic increases by 1/K. Thus, for a 20% duty cycle, the increase in average second harmonic power is 5 times over a constant pump power at the same average power. This property of second harmonic generation becomes very significant when the laser is being pulsed.
A second essential component to consider is the Q-switch with its inherent instabilities. Repetitive Q-switching of an Nd:YAG laser is described in several standard texts such as Solid-State Laser Engineering by W. Koechner, Springer-Verlag, 1988, Second Edition, pp. 402-436. When high average output power is required, the laser is Q-switched at a high repetition rate. However, if the repetition rate is too high, an instability sets in. Instead of producing a train of pulses that are all equal, the laser begins operating so that every second pulse is much smaller. In essence, the large pulses extract all the energy from the laser medium, and when the laser is Q-switched after a large pulse, not enough energy has been stored for the next pulse, and the next pulse is small. The behavior of the laser in this mode becomes more erratic and the high intensity of the large pulses can damage some optical components inside the laser resonator.
Doing simultaneous intracavity second harmonic generation with Q-switching has some advantages. The non-linear behavior of the second harmonic generation (SHG) described earlier will increase the conversion to the second harmonic power at the peak of the large pulses. This SHG non-linearity will thus counteract the Q-switching instability by reducing the amplitude of the larger pulses and equalizing the pulses. However, should the SHG crystal be misaligned and phasematching lost, the large pulses associated with Q-switching will occur and can damage the SHG crystal.
In addition, many elements of a laser system have characteristics that change over time. For instance, an arc lamp ages such that it takes a higher input current to deliver a given output pumping power. Similar aging occurs with other parts of the laser system, such as the gain medium, or other elements that may be in the beam path inside or outside the laser cavity. Thus, in prior art laser systems, adjustments are made at the factory which set a desired input current for driving arc lamp pump sources, using potentiometers or the like, in order to deliver the desired output power. However, as the arc lamp, or other elements of the laser system age, the output power will decrease for that given input current. Since the user cannot adjust that preset current, the laser system suffers a degradation in performance over time.
One technique which is used to monitor the aging of laser systems is to store an output power log in the control system for the laser. This log can then be accessed by service personnel. Based on this stored output power log, the service personnel can make certain assumptions useful in diagnosing problems with the system.
However, the stored output power log used for diagnosis of laser system problems, is a limited information set because of the preset input current to the pump power source.
All the above considerations influence the design of the frequency-doubled laser. For example, in a laser with a Nd:YAG rod 4.0 mm in diameter, 3" long, pumped by a Krypton arc lamp with 6 mm inside diameter and 3" arc length, a KTP crystal 3 mm by 3 mm and 5 mm long, with a spotsize 2.5 times smaller than the spotsize in the Nd:YAG rod, an acousto-optic Q-switch operating at 25 kHz 24W of laser output power at 532 nm may be produced. Pump power required to get this laser output power is about 3.0 to 4.0 kW, and the pump power must stay at this level in both the ready and work modes of operation. The laser design for frequency doubling depends on the thermal focusing produced to keep the laser resonator stable. All thermal transients that may affect stability are avoided by keeping the pump power at full power in both modes. The aim beam during the ready mode is produced by attenuating the full power beam from the laser. To maintain this high pump power, with the attendant waste heat, the laser and particularly the gain medium Nd:YAG rod and krypton (Kr) arc lamp must be cooled with an internal closed-cycle water recycling system, with a water-to-water heat exchanger that is connected to an external water source for cooling. The closed loop system maintains the water clean and pure enough to cool the laser without contaminating the arc lamp or Nd:YAG rod. Typically, 11/2 to 2 gallons per minute are required from the external source at more than 30 psi to remove the waste heat, which in this example can be as much as 4 kW.
As described above, Q-switching the fundamental input power to the non-linear crystal can increase the average second harmonic power produced. Yao, J. Q., et al., "High Power Green Laser by Intracavity Frequency Doubling with KTP Crystal", published in High Power Solid State Lasers (1988), SPIE, Vol. 1021, p. 181, describes another way of increasing the average second harmonic power for a given average pump power; that is to pulse the lamp that pumps the Nd:YAG rod in combination with the Q-switching. Yao, et al. disclose a theoretical and experimental study on a KTP frequency doubled Nd:YAG laser which increases the second harmonic laser output power by "Quasi-CW Pumping". Quasi-CW pumping is described as having a repetition rate of 5 to 100 Hz. Parameters such as minimum current, maximum current, and specific applications are not disclosed or suggested.
Many conventional laser systems which do not have to address the complication of frequency doubling, particularly the Nd:YAG laser systems with output at 1064 nm, have replaced the water-to-water heat exchanger with a water-to-air heat exchanger and are referred to as air-cooled lasers. In surgical use, the conventional air-cooled laser is never turned on at high pump power continuously for a long period of time. Moreover, in systems where the laser goes to high power (and high pump power) only on demand when the surgeon "steps on the footswitch", the average thermal load is not very large. With a reasonable sized water reservoir and water-to-air heat exchanger, these systems can be successfully air-cooled. The advantages of air-cooling are many: 1) no costly water installation in the operating room is required, 2) no inconvenient water hook-ups, 3) the system is much more mobile from operating room to operating room, and 4) power consumption of the system is lower. It is thus extremely desirable to produce a system with second harmonic generation ("SHG") that can be air-cooled. As can be seen from the above discussion, it is desirable to provide a frequency doubled laser system capable of operation in a variety of output modes to optimize performance for a variety of applications of the output beam, and to minimize waste heat.